Fertilizers have been used for some time to provide nitrogen to the soil. Commercial fertilizers can come in a variety of liquid or solid forms. The most widely used and agriculturally important liquid form of nitrogen fertilizer is urea ammonium nitrate (UAN) and the most widely used and agriculturally important solid form is granular urea, a white crystalline solid under normal conditions. Urea is a commonly used nitrogen fertilizer due to its relatively low cost and its high nitrogen concentration (46%). Solid forms of urea, include granular, prilled, pelletized, powdered or dust. Normally, the granular form is somewhat larger in particle size than the prills. Most of the urea-based fertilizer currently used is produced in its granular form.
After urea is applied to soil, it is hydrolyzed to yield ammonia and carbon dioxide. This process is catalyzed by the enzyme urease, which is an extracellular enzyme in the soil. The gaseous products formed by the hydrolysis reaction (ammonia and carbon dioxide) volatilize to the atmosphere and thus, substantial losses from the total amount of the nitrogen applied to the field occur. Accordingly, some solid, water soluble fertilizers can be made slow release by various additives. For example, the hydrolysis process can be considerably decelerated by applying enzyme inhibitors, specifically urease or nitrification inhibitors with urea. Examples of urease inhibitors are the thiophosphoric triamide compounds disclosed in the U.S. Pat. No. 4,530,714, including N-(n-butyl)thiophosphoric triamide (NBPT). NBPT is used in a number of agricultural products, such as AGROTAIN® and AGROTAIN ULTRA® (see e.g. U.S. Pat. No. 5,698,003) and SUPER N® (see e.g. U.S. Pat. No. 5,364,438) and SUPER U®, UFLEXX® and UMAXX® (see e.g. U.S. Pat. No. 5,352,265).
Industrial grade N-(n-butyl)thiophosphoric triamide (NBPT) is a solid, waxy and sticky compound, that decomposes in water and at elevated temperatures. Accordingly, its direct application onto urea particles is very difficult. In some applications, PERGOPAK® M by the Albemarle Corporation (which is made by the process disclosed in U.S. Pat. No. 6,936,078) has been used as a carrier for NBPT (see U.S. Patent Publication 2007/0157689). NBPT is deposited into the PERGOPAK® M by first dissolving the NBPT in NMP and then drying the NBPT and PERGOPAK® M mixture to form a solid. This solid is then blended with granulated urea. An alternative form of this product can be made by applying the molten NBPT directly to the PERGOPAK® M and then subsequently blending this with granulated urea.
However, the combination of NBPT with PERGOPAK® M can result in several problems making its use difficult. The combination can form large clumps which must be filtered out before use. This leads to poor product yield. The combination can also have difficulty flowing through equipment, which leads to poor consistency of application levels on urea. The combination is also dusty and has an undesirable odor. These issues can lead to variations in NBPT ratios when formulating the mixture of NBPT and PERGOPAK® M with granulated urea. Because of variations in the NBPT levels, formulators may need to add a significant excess of the NBPT/PERGOPAK® M formulation to form a commercial, granulated urea formulation. In addition, there are also safety challenges in managing the dust levels at formulation facilities. Further, the mixture of NBPT and PERGOPAK® M has limited long-term stability and can typically only be stored in containers smaller than or including a fiber drum in order to minimize decomposition.
Accordingly, there remains a need for new compositions and methods of improving the properties of urea-based fertilizers coated with other active agents. The present invention as described herein addresses this and other needs by providing a formulation of urease or nitrification inhibitors with a urea-formaldehyde polymer as disclosed in U.S. Pat. No. 6,936,681. The formulation of the present invention significantly improves the flow of the mixture comprising the urease inhibitor, thereby reducing dust, increasing bulk density and providing more uniform formulated product.